Am I going to earn Pravda Coin for this action? Only if other people who possess a significant number of Pravda Coin also upvote the same content.
Ok, so in the end, you incentivize people to vote according what other people have voted or would vote, no?
Yup. People are incentivized to vote on content that they believe other people will upvote in the future. If the network is intended to evaluate credibility then the best strategy for the upvoter is to upvote content they believe other people will evaluate as "credible." The best strategy for accomplishing that is to upvote content they believe is credible.